European Council President Donald Tusk warned Britain on Thursday that its plan to leave the EU’s customs union and single market on Brexit could mean a return to a “hard border” on the island of Ireland.

Addressing a business conference in Brussels before leaving for lunch in London with British Prime Minister Theresa May, the EU summit chair said an EU proposal on Wednesday to incorporate Northern Ireland within a “common regulatory area” with the EU was the best option to avoid border friction -- but he would be asking in London if Britain could propose something better.

“Until now, no one has come up with anything wiser than that,” Tusk told the Business Europe event. “In a few hours, I will be asking London whether the UK government has another idea that will be as effective in preventing a hard border between Ireland and Northern Ireland.”

Tusk also confirmed that he will distribute negotiating proposals next week for a future trade relationship with Britain. That will follow May’s expected announcement of her proposals on Friday.

But, Tusk warned, May’s “red lines” of leaving the single market and customs union meant that some friction in EU-UK trade would be inevitable.

“There can be no frictionless trade outside of the customs union and the single market. Friction is an inevitable side effect of Brexit, by nature,” he said.

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